This invention relates to an injection molding machine comprising a mold for molding a resin material into a molded product and an ejector mechanism for ejecting the molded product out of the mold and, in particular, to a calibration method and a control device for an ejector mechanism.
An ejector mechanism of the type described comprises an ejector member driven by a servo motor. In order to eject a molded product, an advance operation is carried out in which the ejector member is advanced or moved forward. After ejection of the molded product, a reverse operation is carried out in which the ejector member is returned or moved backward. Generally, the servo motor is coupled to an encoder. The encoder produces a pulse signal having a variable number of pulses which varies in correspondence to the number of rotation of the servo motor. The number of pulses contained in the pulse signal is counted by an updown counter as a count value. The count value is related to a travelling distance of the ejector member and is therefore used to monitor a stop position where the ejector member is stopped after the molded product is ejected and the ejector member is reversed. Such monitoring of the stop position is required because, in order to carry out an accurate ejection, it is important to precisely adjust the stop position where the ejector member is stopped after the reverse operation. Thus, the encoder serves as a position detector for the ejector member.
In the ejector mechanism described above, it is necessary to carry out calibration by the use of a detector such as a proximity switch when a control power supply is turned on, namely, when an injection molding process is started. For this purpose, the detector is located in an intermediate position of a travelling path of the ejector member. Briefly speaking, in a conventional calibration method, the reverse operation of the ejector member is carried out at the start of the injection molding process. In the reverse operation, the ejector member is moved backward to a position where the detector is turned on to produce an ON signal. In response to the ON signal, the updown counter clears the count value into a reset value of zero. Thus, the ejector mechanism is calibrated.
In the above-mentioned conventional calibration method, however, a location of the detector must be modified every time when the mold is exchanged for another mold.